What you are describing will work. There's enough give in the 2" PVC that you can use 90-degree elbows if you make sure the top end of the 8" run is a couple inches lower than the bottom to provide your 1/4" of fall per foot of run. If you've got more room and want to slope it more, that's fine too. Use banded no-hub couplings to connect the PVC and the cast iron as others have suggested.

For what it's worth, the connection that you can't see under the dirt is probably a wye and a 45 degree elbow (or a equivalent combo fitting). Square tees aren't generally used (or permitted by code) in drain lines because they don't permit the water to flow smoothly around the corner.